Method for production of novolac fibers

ABSTRACT

A novolac melt is fiberized to produce a thermoplastic, uncured novolac fiber, and the novolac is curved by heating the fiber in a formaldehyde environment in the presence of an acid catalyst to obtain an infusible, cured novolac fiber.

United States Patent Economy et al.

METHOD FOR PRODUCTION OF NOVOLAC FIBERS Inventors: James Economy,Eggertsville, N.Y.;

Rodger A. Clark, Indianapolis, Ind.

Assignee: The Carborundum Company,

Niagara Falls, N.Y.

Filed: Nov. 4, 1970 Appl. No.: 87,002

Related US. Application Data Division of Ser. No. 710,292, March 4,1968, Pat. No. 3,650,102.

US. Cl. ..264/83, 264/164, 264/176 F, 264/210 F, 264/236, 264/347 Int.Cl ..B29c 25/00, D01f 5/12 Field of Search ..264/176 F, 83, 236, 347,164, 264/210 F; 260/59 References Cited UNITED STATES PATENTS 9/1937Cherry ..260/59 Reaction Spinning of Fibers by Pohl, HA. TextileResearch Journal pp. 473-477 (1958) Primary ExaminerJay H. WooAtt0mey--K. K. Brownell [57] ABSTRACT A novolac melt is fiberized toproduce a thermoplastic, uncured novolac fiber, and the novolac iscurved by heating the fiber in a formaldehyde environment in thepresence of an acid catalyst to obtain an infusible, cured novolacfiber.

16 Claims, 1 Drawing Figure METHOD FOR PRODUCTION OF NOVOLAC FIBERS Thisis a division of application Ser. No. 710,292, filed Mar. 4, 1968, nowUS. Pat. No. 3,650,102.

The present invention relates to fibers from novolacs and to a methodfor their production.

In accordance with the method of the invention, a novolac is melted andformed into a fiber by any convenient fiberizing means to produce athermoplastic, uncured novolac fiber. The novolac is then renderedinfusible by heating the fiber in a formaldehyde environment in thepresence of an acid catalyst at a temperature and for a time sufficientto effect curing, i.e., advancement of the molecular weight of thenovolac to obtain an infusible, cured novolac fiber.

Cured novolac fibers according to the invention have various highlydesirable properties which will be elaborated upon hereinafter, theseproperties rendering the fibers, in suitable forms, useful in a widevariety of practical applications.

The drawing schematically illustrates apparatus which is useful infiberizing a novolac in carrying out the method of the invention. I

The first step in carrying out the method of the invention is tofiberize the novolac selected as the starting material. This isconveniently accomplished with the use of apparatus such as thatillustrated in the drawing, which comprises a vessel 2 made of anysuitable material such as stainless steel mounted on any suitablesupport 4. The vessel 2 has an orifice 6 at its bottom, and its exterioris surrounded by electric wire heating coils 8 connected to anadjustable source of electricity (not shown) whereby a controlled amountof heat may be imparted to the vessel and its contents. The apparatusfurther comprises a spool 10 attached to the shaft 12 of a variablespeed electric motor 14 mounted beneath the vessel 2.

In operating the apparatus, the novolac selected as the startingmaterial is placed in the vessel 2 and heat is applied to the vessel bymeans of the heating coils 8 to melt the novolac and maintain the melt15 at the desired temperature. The melt flows out of the orifice 6 andis attenuated into a fiber 16 as a result of being drawn downward byrevolution of the spool 10 to which the fiber is attached. The fiber,which solidifies upon contacting the atmosphere and cooling, is wound upon the spool. The drawing rate may be regulated by the speed at whichthe spool 10 is driven by the variable speed motor 14.

EXAMPLE 1 To 4,880 g of a mixture consisting of 77 percent phenol, 8.7percent formaldehyde and 14.3 percent water is added 8 g of 95 percentsulfuric acid. The mixture is heated to 70C whereupon anovolac-producing reaction begins to occur. Over a 15 minute period,1,640 g of an aqueous 37 percent formaldehyde solution is added, and themixture is allowed to reflux for 3 hours. A solution of 7 g of sodiumhydroxide in 50 ml of water is added to neutralize the sulfuric acid.The mixture is then vacuum distilled, to remove the water present, at atemperature gradually increasing to a final temperature of 150C, and thenovolac which remains is allowed to cool to about 100C.

The novolac is stirred vigorously with 2 l. of water and the mixture isallowed to settle, the water then being decanted and discarded, thiswater wash serving to reduce free phenol, sodium sulfate, and lowmolecular weight novolac fractions. The water wash is repeated twice andthe novolac is allowed to cool to ro o m temperature, solidifying. Thenovolac has a Mn (number-average molecular weight) of about 850 and aviscosity at 130C of about 68,800 cps.

EXAMPLE 2 Apparatus is assembled as shown in the drawing, employing avessel having an orifice with an inner diameter of 1.9 mm. A 100 gportion of a novolac prepared as in Example 1 is placed in the vesseland heated to C, and the resulting melt is drawn into a fiber at a rateof about 3,000 feet (915 meters) per minute. The

. drawn fiber is wound up on the spool, which is made of a suitablematerial such as graphite or polypropylene which is not affected by thereagents subsequently to be employed in curing the novolac.

The resulting uncured novolac fiber has a slightly elliptical crosssection and an average diameter of about 12 microns. The fiber isthermoplastic, but weak and brittle, and has a glassy or amorphousstructure. Birefringence suggests that the molecules in the fiber tendto be oriented in the direction of the axis of the fiber.

The fiber-containing spool is immersed at room temperature in a mixtureof 500 ml of 36 percent hydrochloric acid and 500 m1 of an aqueous 37percent formaldehyde solution; i.e., an aqueous solution containing 18%HCl and 18.5 percent formaldehyde. The temperature of the solution isbrought to 40C, then increased gradually from 40 to 60C over a period of3 hours, and finally increased from 60 to 100C over a period of 1 hour.It is to be noted that no fusion of the fiber occurred. The fiber isremoved, washed with water, and dried in air at about 100C.

The properties of randomly selected samples of the resulting infusible,cured novolac fiber are determined, with the following results. Tensilestrength ranges from about 15,000 to about 30,000 psi (1,050 to 2,100kg/sq cm), averaging 22,000 psi (1,540 kg/sq cm). Elastic modulus rangesfrom 0.48 X 10 to 1.04 X 10 psi (0.034 X 10 to 0.073 X 10 kg/sq cm),averaging 0.76 X 10 psi (0.053 X 10 kg/sq cm). Break elongation rangesfrom 3.2 to 5.4 percent, averaging 4.0 percent.

Fracture energy ranges from 0.020 to 0.058 inch gm/lOOy. (0.051 to 0.147cm gm/100p averaging- 0.036 inch gm/lOOy. (0.091 cm gm/100u EXAMPLE 3The cured novolac fiber prepared in Example 2 is further cured byheating it for 6 hours at C in a nitrogen atmosphere. After cooling, theproperties of various portions of the cured novolac fiber are measured,with the following results. Tensile strength ranges from 20,000 to34,000 psi (1,400 to 2,400 kg/sq cm), averaging 26,000 psi (1,800 kg/sqcm). Elastic modulus ranges from 0.63 X 10 to 1.06 X 10 psi (0.044 X 10to 0.074 X 10 kg/sq cm), averaging 0.8 X 10 psi (0.056 X 10 kg/sq cm).Break elongation ranges from 7.1 to 19.1 percent, averaging 11.6percent.Fracture energy ranges from 0.079 to 0.315 inch gm/lOOu (0.20 to 0.80 cmgm/100u averaging 0.184 inch gm/lp. (0.47 cm gm/100p.

It may be noted that a slight increase in tensile strength and a markedincrease in break elongation and fracture energy is effected by thefurther curing.

EXAMPLE 4 Apparatus is assembled as shown in the drawing, employing avessel having an orifice with an inner diameter of 1.9 mm. A 30 gquantity of a phenol formaldehyde novol ac similar to that prepared inExample 1 but having a Mn of 690 and a viscosity of 13,600 cps at 130Cis placed in the vessel. The novolac is heated to 105C, and theresulting melt is drawn into a fiber at a rate of about 600 feet (185meters) per minute, the fiber being wound up on the spool. The fiber hasa diameter of about 15 microns.

The fiber is removed from the spool and suspended in a vertical glasstube 12 inches (30 cm) long having an inner diameter of 2 inches (5 cm).The tube may be heated from, an external source and is adapted to permitthe flowof a gas therethrough.- A gaseous mixture of hydrogen chlorideand formaldehyde is passed upwardly through the tube at a rate of about50 m1 of each per minute. As the gas flow is continued, the temperature,as measured by thermocouples in the tube, is increased from 25 to 50Cover a period of 1.5 hour, then from 50 to 65C over a period of 1.5hour, and finally from 65 to 105C over a period of 1 hour.

The resulting infusible, cured novolac fibers have an average tensilestrength of 20,000 psi (1,400 kg/sq cm), an average break elongation of3.7 percent, and an average elastic modulus of 0.70 X psi (0.049 X 10kg/sq cm).

After further curing the fibers by heating them at 150C for 6 hours in avacuum (about mm of Hg), they have an average tensile strength of 25,500psi (1,780 kg/sq cm), an average break elongation of 10.5 percent, andan average elastic modulus of 0.75 X 10 psi (0.052 X 10 kg/sq cm).

EXAMPLE 5 Apparatus is assembled as shown in the drawing, employing avessel having an orifice with an inner diameter of 1.9 mm. A 30 gquantity of a m-cresol formaldehyde novolac having a Mn of 970 and aviscosity of about 35,000 cps at 165C is placed in the vessel. Thenovolac is heated to 165C and the resulting melt is drawn into a fiberat a rate of 600 feet 185 meters) per minute, the resulting fiber havinga diameter of microns.

The fiber is cut from the spool and cured to produce an infusible fiber,employing a vertical tube and gaseous hydrogen chloride and formaldehydeas described in Example 4. The temperature during the curing cycle israised from to 130C over a period of 20 minutes. For further cure, thetemperature is further increased from 130 to 190C over a period ofminutes while maintaining the flow of hydrogen chloride andformaldehyde.

Measurements on the resulting cured novolac fibers show that they havean average tensile strength of 20,000 psi 1,400 kg/sq cm), an averagebreak elongation of 8 percent, and an average elastic modulus of 0.7 X10 psi (0.049 X 10' kg/sq cm).

EXAMPLE 6 One hundred g of a diphenyl oxide modified phenol formaldehydenovolac available from The Dow Chemical Company under the designationET8231,200 is fiberized using apparatus as shown in the drawingemploying a vessel with an orifice having an inner diameter of 1.9 mm.The novolac has a diphenyl oxide to phenol mole ratio of 1:1, a Mn of1,200, and a viscosity of 40,000 cps at C. Fiberization is carried outat 110C at a rate of 600 feet (185 meters) per minute to produce a fiberhaving a diameter of 14 microns, the fiber being wound up on the spool.

The fiber, on the spool, is immersed at room temperature in an aqueoussolution containing 18 percent HCl and 18.5 percent formaldehyde for 16hours. The temperature is then increased over a period of 6 hours fromroom temperature to 100C, where it is held for 2 hours. The infusiblefiber is removed, washed with water, and dried in air at 100C, thenfurther cured by heating at C for 6 hours in a nitrogen atmosphere. Theresulting cured novolac fiber has an average tensile strength of 27,000psi (1,890 kg/sq cm), an average break elongation of 1 1.5 percent, andan average elastic modulus of 0.65 X 10 psi (0.045 X 10 kg/sq cm).

EXAMPLE 7 A mixture of 540 g of m-cresol, 540 g of p-cresol, 730 g of anaqueous 37 percent formaldehyde solution and 2.16 g of oxalic acid isheated at refluxing temperature 1.5 hour to produce a m,p-cresolformaldehyde novolac. The reaction mixture is neutralized with 2N NaOH.The excess formaldehyde, unreacted cresols, and water are removed bydistillation at atmospheric pressure and then distillation in vacuum(about 15 mm of Hg) up to a finaLtemperature of C. The resulting novolachas a Mn of 720 and a viscosity-of about 30,000 cps at 130C. I

The novolac is fiberized as described in Example 2 at a temperature of130C and a rate of 600 feet meters) per minute to produce a fiber havinga diameter of 25 microns. The fiber is cured as in Example 6 to producean infusible, cured novolac fiber.

EXAMPLE 8 Fifty g of phenol formaldehyde novolac prepared as in Example1 and 50 g of p-phenyl phenol novolac available fromArcher-Daniels-Midland Company under the trade name Syncoat 618 aremelted and mixed together at about 130C, at which temperature themixture has a viscosity of 35,000 cps. The melt is fiberized as inExample 2 at a temperature of 130C and a rate of about 600 feet (185meters) per minute to produce a fiber having a diameter of 15 microns.

The fiber is cured as in Example 6 to produce an infusible, curednovolac fiber.

As may be seen from the examples, fibers may be prepared according tothe invention from any of a wide variety of novolacs. The term novolacrefers to a conv densation product of a phenolic compound withforgroups. The phenolic compound may be phenol, or phenol wherein one ormore of the non-hydroxylic hydrogens are replaced by any of varioussubstituents attached to the benzene ring, a few examples of which arethe cresols, phenylphenols, 3,5-dialkylphenols, chlorophenols,resorcinol, hydroquinone, phloroglucinol, and the like. The phenoliccompound may instead be a naphthol or a hydroxyphenanthrene or anotherhydroxyl derivative of a compound having a condensed ring system.

For purposes of the present invention, any fusible novolac which iscapable of further polymerization with a suitable aldehyde may beemployed for the production of fibers. Stated another way, the novolacmolecules must have two or more available sites for furtherpolymerization. Apart from this limitation any novolac may be employed,including modified novolacs, i.e., those in which a non-phenoliccompound is also included in the molecule, such as the diphenyl oxidemodified phenol formaldehyde novolac used in Example 6. Mixtures ofnovolacs may be employed, as in Example 8. Novolacs containing more thanone species of phenolic compound may be employed, such as the meta andpara-cresol novolac in Example 7'.

Novolacs generally have a number-average molecular weight in the rangefrom about 500 to about 1,200, although in exceptional cases a molecularweight as low as 300 or as high as 2,000 or more may occur. Unmodifiedphenol formaldehyde novolacs usually have a number-average molecularweight in the range from about 500 to about 900, most of thecommercially available materials falling within this range.

Novolacs of any molecular weight may be employed in carrying out themethod of the invention. However, there are generally certaindisadvantages to employing a novolac having a molecular weight at theextreme upper or lower end of the molecular weight range for the type ofnovolac under consideration. When a very high molecular weight novolacis used, it is usually necessary to resort to a somewhat higherfiberizing temperature than would otherwise be necessary, in order toachieve a melt viscosity which is sufficiently low to permitfiberization. It is frequently found that, at this higher temperature,there is a tendency of the novolac to gel, thus interfering with properfiberization. On the other hand, when a very low molecular weightnovolac is used, the temperature at which such novolac softens andbecomes tacky is usually comparatively low, and it is thereforenecessary to cure the fiberized novolac at a very low temperature toavoid adherence and/or deformation of the fibers. It is usuallyundesirable to employ such low curing temperatures since, as will beseen, the curing rate increases dramatically with increasingtemperature, and low temperature curing entails the practicaldisadvantage of a prolonged curing cycle. Balancing the foregoingfactors, it is generally preferred to employ a novolac having amoderately high molecular weight for the type of novolac underconsideration to permit curing in a reasonable time without adherenceand/or deformation, but to avoid the extreme upper end of the molecularweight range to minimize problems in fiberizing due to gelling.

Of the many types of novolacs which are known, it is usually preferredto use a phenol formaldehyde novolac such as employed in Examples 2 and4, since they are usually the least expensive.

When employing fiberizing apparatus such as that shown in the drawingfor melt spinning the selected novolac, fibers may be produced having arather uniform diameter. A wide range of diameters may be produced,fibers having been made with a diameter less than 4 microns and greaterthan microns. The fiber diameter depends primarily upon two factors, thedrawing rateand the flow rate of the melt through the orifice. The fiberdiameter decreases as the drawing rate is increased, and increases asthe flow rate of the melt is increased. The flow rate of the meltdepends primarily upon the diameter and length of the orifice and theviscosity of the melt, increasing as the orifice diameter is increased,decreasing as the length of the orifice is increased, and increasing asthe viscosityof the melt is decreased. An increase in flow rate may alsobe effected, if desired, by applying pressure to the melt whereby toforce it through the orifice. The orifice must-be far enough above thespool to give the newly formed fiber ample time to cool and solidify.

When the various factors involved are properly taken into account,satisfactorY fibers may generally be drawn from a novolac melt having aviscosity of from about 5,000 to about 70,000 cps, although it isgenerally preferred to operate in the range from about 20,000 to about50,000 cps. Accordingly, the temperature of the novolac melt should besuch as to result in such a viscosity with the particular novolac beingused. As noted above, when the novolac has an extremely high molecularweight for the particular type of novolac being employed, an excessivelyhigh temperature may be required to obtain a suitable fiberizingviscosity and some gelling may therefore tend to occur. Thus, it isusually preferred to avoid the use of such high molecular weightnovolacs. 1

Other conventional modifications and methods may be employed infiberizing the novolac. For example, the melted novolac may be extRudedunder pressure while belng drawn and, if desired, the novolac melt mayfirst be forced through a filter under pressure to remove any solidimpurities and thereby improve the quality of the fibers. Instead ofdrawing, a blowing method may be used whereby a novolac melt is allowedto drop in a thin stream into the path of a blast of air which fiberizesthe stream. The blowing method produces staple fibers which varyconsiderably in length and diameter.

The uncured novolac fibers of the inventiOn are thermoplastic, butgenerally rather weak and brittle, and are usually colorless or lightamber. Birefringence indicates that there is some tendency for thenovolac molecules to orient in the direction of the axis of the fiber.As already noted, fibers have been made with diameters of less than 4microns up to more than [00 microns. By the blowing method, fibers withdiameters as low as about 0.1 micron may be prepared, and fibers aslarge as about 300 microns in diameter may be drawn.

Curing of the novolac to render the fiber infusible is effected byheating the uncured novolac fiber in a liquid or gaseous formaldehydeenvironment in the presence of an acid catalyst. It appears that thecuring mechanism involves the diffusion of the formaldehyde into thefiber and reaction of the novolac and formaldehyde to bring aboutpolymerization of the novolac molecules.

One means of effecting the presence of the acid catalyst during thecuring is to incorporate a small amount of a suitable acid such assulfuric, phosphoric or oxalic acid in the novolac melt prior tofiberization, thereby obtaining uncured novolac fibers containing acatalytic quantity of the acid. The fibers may then be heated in aliquid or gaseous formaldehyde-containing environment. There are,however, two disadvantages to following this procedure. First, thepresence of the acid in the novolac melt may increase the tendencytoward gel formation in the melt. Secondly, prolonged washing of thecured novolac fibers is often required to remove the acid.

Accordingly, it is preferred to effect curing by heating the uncurednovolac fibers in an environment containing both the formaldehyde andthe acid. The environment may be gaseous, as in Examples 4 and 5, but ispreferably liquid as in Examples 2 and 6, i.e., a solution of the acidand formaldehyde. Liquid is preferred because of the greater rapidity ofheat and material transport to the fibers, especially the fibers in theinterior portions of a bundle of fibers being cured, and also becausehigher concentrations of formaldehyde and acid may be achieved byemploying a solution thereof.

When a solution is employed for the curing step, any of a wide varietyof acids may be used as the catalyst, including mineral acids such ashydrochloric, sulfuric and phosphoric acids, and organic acids such asoxalic acid. Hydrochloric acid has been found to be eminently suitable.Water is the solvent of choice, although other liquids may be employed,provided that they do not adversely affect the fiber and are capable ofdissolving the formaldehyde and acid. It is preferred that the solutioncontain from about 12 percent to about 18 percent each of the acid andformaldehyde. As little as l percent of each will suffice, but asomewhat longer curing cycle is then usually required. More than 18percent of either or both may be used but does not appear to offer anyadvantage.

When curing is carried out in a gaseous environment, any gaseous acidsuch as hydrogen bromide or hydrogen chloride may be employed, thelatter being particularly suitable. The formaldehyde may conveniently begenerated by heating paraformaldehyde. The gaseous atmosphere maycontain as little as about percent-formaldehyde up to as much as 99percent, by volume, and from about 1 percent to about 90 percent byvolume of the acid. If desired, the atmosphere may also'contain adiluent such as nitrogen or other inert gas, but air should be excludedto minimize the possibility of side reactions taking place.

In either a gaseous or liquid environment, the rate of curing increasesdramatically with increasing temperature. Table I shows the approximatecuring time necessary at various temperatures for phenol formaldehydenovolac fibers.

TABLE 1 Tem nature A roximate Curin Time ("C) (n iiimtes) g I 8000 504200 75 600 l00 100 I25 150 15 175 6 200 3 It may be noted that it ispossible to cure the novolac fibers at room temperature (25C) but it ishighly impractical to do so because of the time required. In theinterest of minimizing the curing time, it is preferred to cure thefibers at the highest temperature at which adherence and/or deformationof the fibers does not occur. In general, the lower the molecular weightof the novolac, the lower the temperature at which these occur.Therefore, it is usually preferred not to use extremely low molecularweight novolacs, thereby avoiding the need for very low curingtemperatures and the attendant slow curing rates.

It is usually desirable to carry out the curing cycle at graduallyincreasing temperatures. Initially, a temperature is employed at whichadherence and/or deformation does not occur. At this stage, the outerportion of the fiber begins to cure, forming a shell. Thereupon, thetemperature may be raised as necessary to complete the cure, the shelleliminating any-problems due to fusion which might otherwise occur. Suchcuring cycles are illustrated in Examples 2, 4, 5 and 6.

The curing time must be sufficiently long to render the uncured novolacfiber infusible. Once such infusibility has been achieved, furthercuring is unnecessary for purposes of the invention. However, it hasoften been found that further curing tends to enhance certain propertiesof the fibers, particularly elongation and fracture energy, and to alesser degree the tensile strength. Such improvements may be seen fromExamples 2 and 3, and also from Example 4. The further curing may becarried out by continued heating in the original curing environment, asin Example 5. Alternatively, the infusible fibers may be removed fromthe original environment and further cured by heating in a non-oxidizingatmosphere which serves to minimize side reactions. Preferably, an inertatmosphere such as nitrogen or a vacuum is employed. This method isillustrated in Examples 3, 4 and 6.

The infusible, cured novolac fibers of the invention possess a number ofhighly desirable properties, these properties being substantially thesame regardless of the type of novolac employed or the molecular weightthereof. They are remarkably resistant to heat and flame, beinginfusible and non-flammable. They are also substantially unaffected bymany acids and are insoluble in organic solvents.

The mechanical properties of the cured novolac fibers vary with thefiber diameter, and the tensile strength and break elongation bothincrease markedly with decreasing diameter. Considering a cured phenolformaldehyde novolac fiber with a diameter of 14-15 microns, typicalproperties would be: tensile strength, l5,000-35,000 psi (l,050-2,450kg/sq cm); elastic modulus, 0.5-1.2 X 10 psi (0.035-0084 X 10 kg/sq cm);break elongation, 3--20 percent; fracture energy, 0.02-0.30 inchgm/lOOp. 2 (0.05-0.76 cm gm/l00a Properties as high as the followinghave been observed for such a fiber: tensile strength, 60,000 psi (4,200kg/sq cm); elastic modulus, 1.5 X 10 psi (0.] X 10 kg/sq cm breakelongation, 44 percent; fracture energy,'0.46 inch gm/lOOp. (1.17 cmgm/l00p.

Various conventional textile techniques may be employed to process thecured novolac fibers into a variety of useful forms. The fibers, whenprepared by drawing, are initially in continuous form, and may be curedin such form or cut from the spool and cured in the form of a staplefiber. Staple fiber may also be produced by a conventional blowingmethod. in continuous form, the fibers are useful for a wide variety ofpurposes in the same manner as continuous fibers of other well-knownmaterials. Considering the staple form, this may be chopped into shortlengths and made into paper by conventional means. Alternatively, thestaple fiber may be carded to produce a fluffy web, which may beprocessed by needling to obtain needled felt or which may be precessedwith a resin binder to make resin bonded felt. The fluffy web mayinstead by divided into strips which are slightly twisted to form rovingfrom which yarn may be formed which in turn may be woven into cloth.

The infusible, cured novolac fibers, in these various forms, may beemployed for a wide range of purposes. By virtue of their resistance toheat and chemicals, paper and cloth made from such fibers are wellsuited to use as filters, for example in the filtration of hot gases asin air pollution control, and in the filtration of concentrated sulfuricor phosphoric acid, even at temperatures of 250C or higher. Cloth madefrom such fibers is well suited to use in making flame protectiveclothing, and is useful as a backing for coated abrasive products byvirtue of its resistance to heat. The fibers, in suitable forms, may beused as thermal insulation and for the production of ablation materials.

Except as otherwise specified, percentages herein refer to percentagesby weight.

Molecular weights of novolacs refer to numberaverage molecular weights.Molecular weight determinations were carrier out by vapor phaseosmometry.

Viscosities have been stated herein in centipoises. However, in allcases the viscosities were originally determined as apparent viscositiesin 1b sec/in with a Model G1 D&R Melt Indexer, and converted tocentipoises by multiplying by a factor of 6.88 X 10 as reported in theliterature.

We claim:

1. A method for the production of an infusible, cured novolac fibercomprising forming a melt of a fusible novolac which is capable ofpolymerization with a suitable aldehyde, fiberizing said melt to form athermoplastic, uncured novolac fiber, and curing said uncured novolacfiber by heating it at a suitable temperature and for a sufficient timein a formaldehyde environment in the presence of an acid as a catalyst,to render it infusible.

2. A method as set forth in claim 1 wherein said melt is fiberized byblowing a thin falling stream of said melt.

'3. A method as set forth in claim 1 wherein said melt is fiberized bydrawing it through an orifice.

4. A method as set forth in claim 3 wherein said melt has a viscosity offrom about 5,000 cps to about 70,000 cps.

5. A method as set forth in claim 3 wherein said melt has a viscosity offrom about 20,000 cps to about 50,000 cps.

6. A method as set forth in claim 1 wherein said novolac is a phenolformaldehyde novolac.

7. A method as set forth in claim 1 wherein said acid is present in saiduncured novolac fiber in a catalytic quantity.

8. A method as set forth in claim 1 wherein said uncured novolac fiberis cured by heating it in a solution of formaldehyde and an acid.

9. A method as set forth In claim 8 wherein said solution is an aqueoussolution and said acid is hydrogen chloride.

10. A method as set forth in claim 9 wherein said formaldehyde and saidhydrogen chloride are each present in an amount of from about 12 percentto about 18 percent.

11. A method as set forth in claim 1 wherein said uncured novolac fiberis cured by heating it in a gaseous atmosphere containing an acid andfrom about 10 percent to about 99 percent by volume of formaldehyde.

12. A method as set forth in claim 11 wherein said acid is hydrogenchloride.

13. A method as set forth in claim 1 including the step of furthercuring said infusible fiber by heating it in a nonoxidizing atmosphere.

14. A method as set forth in claim 13 wherein said atmosphere isnitrogen.

15. A method as set forth in claim 13 wherein said atmosphere is avacuum.

16. A method for the production of an infusible, cured novolac fiberaccording to claim 1 wherein said novolac is a phenol formaldehydenovolac, said melt has a viscosity of from about 5,000 cps to about70,000 cps, and said melt is fiberized by drawing it through an orificeto form an uncured novolac fiber having a diameter of from less thanabout 4 microns to about 300 microns, curing said uncured novolac fiberby heating it in an aqueous solution containing at least about 1 percenteach of formaldehyde and an acid to render said fiber infusible, andfurther curing the resulting infusible fiber by heating it in anon-oxidizing atmosphere selected from the group consisting of nitrogenand a vacuum.

2. A method as set forth in claim 1 wherein said melt is fiberized byblowing a thin falling stream of said melt.
 3. A method as set forth inclaim 1 wherein said melt is fiberized by drawing it through an orifice.4. A method as set forth in claim 3 wherein said melt has a viscosity offrom about 5,000 cps to about 70,000 cps.
 5. A method as set forth inclaim 3 wherein said melt has a viscosity of from about 20,000 cps toabout 50,000 cps.
 6. A method as set forth in claim 1 wherein saidnovolac is a phenol formaldehyde novolac.
 7. A method as set forth inclaim 1 wherein said acid is present in said uncured novolac fiber in acatalytic quantity.
 8. A method as set forth in claim 1 wherein saiduncured novolac fiber is cured by heating it in a solution offormaldehyde and an acid.
 9. A method as set forth in claim 8 whereinsaid solution is an aqueous solution and said acid is hydrogen chloride.10. A method as set forth in claim 9 wherein said formaldehyde and saidhydrogen chloride are each present in an amount of from about 12 percentto about 18 percent.
 11. A method as set forth in claim 1 wherein saiduncured novolac fiber is cured by heating it in a gaseous atmospherecontaining an acid and from about 10 percent to about 99 percent byvolume of formaldehyde.
 12. A method as set forth in claim 11 whereinsaid acid is hydrogen chloride.
 13. A method as set forth in claim 1including the step of further curing said infusible fiber by heating itin a nonoxidizing atmosphere.
 14. A method as set forth in claim 13wherein said atmosphere is nitrogen.
 15. A method as set forth in claim13 wherein said atmosphere is a vacuum.
 16. A method for the productionof an infusible, cured novolac fiber according to claim 1 wherein saidnovolac is a phenol formaldehyde novolac, said melt has a viscosity offrom about 5, 000 cps to about 70,000 cps, and said melt is fiberized bydrawing it through an orifice to form an uncured novolac fiber having adiameter of from less than about 4 microns to about 300 microns, curingsaid uncured novolac fiber by heating it in an aqueous solutioncontaining at least about 1 percent each of formaldehyde and an acid torender said fiber infusible, and further curing the resulting infusiblefiber by heating it in a non-oxidizing atmosphere selected from thegroup consisting of nitrogen and a vacuum.